1. Field of the Invention
The present invention relates to a gas permeable (hereinafter sometimes referred to as “porous”) alumina sintered body having communicating pores, and a method for producing the same. More particularly, it relates to a porous alumina sintered body in which characteristics of the communicating pores, such as pore diameter and porosity, are accurately controlled to desired ranges and which is suitably usable as various gas permeable industrial materials, e.g., electrode protective layers for measurement electrodes of gas sensors, filters of dust collectors, etc., and a method for efficient production of the alumina sintered body.
2. Description of the Prior Art
Porous alumina sintered bodies are suitably used as various gas permeable industrial materials, e.g., electrode protective layers for measurement electrodes of gas sensors, filters of dust collectors, etc. The gas sensors include, for example, oxygen sensors used for measuring oxygen concentration in exhaust gases of automobiles to detect the combustion state in the engines (JP-A-9-68515).
Recently, for the improvement of performance of automobiles, it is attempted not only to enhance performance of engines, but also to add various additives such as phosphorus, zinc, magnesium and calcium to engine oil or gasoline. However, if these additives mingle into exhaust gases, they stick to the surface of oxygen sensor to clog the communicating pores of the diffusion resistant layer or stick to the measuring electrode in a measuring chamber to deteriorate the electrode, resulting in reduction of output of the sensor or reduction of responsiveness. Moreover, the porous alumina sintered body is produced, for example, by coating and drying a slurry having a proper viscosity and thereafter sintering the coat, and there are caused considerable variations in pore diameter and porosity of the resulting alumina sintered body (electrode protective layer), and the gas output yield is not stabilized. Furthermore, since the gas to be measured must permeate the electrode protective layer and reach the electrode to output an electrical signal, the layer is required to have uniform porosity higher than a specific value, and, besides, in order to inhibit deterioration of the measuring electrode caused by the effect of the additives in the gas, the layer is required to have a uniform pore diameter smaller than a specific value. In other words, the electrode protective layer is required to have intermediate physical properties between those of porous body and dense body.
At present, there have not yet been obtained porous alumina bodies in which pore characteristics of the communicating pores, such as pore diameter and porosity, are accurately controlled to desired ranges.